If the phone was locked to AT&T, it wouldn’t work. You will have to unlock it to use on Koodo network.

Unlocking the phone would, of course, be necessary.  My question is ‘will it be able to work on the Koodo/Telus network?’.

Yes, North American version should work fine. 

AT&T’s radio frequency bands are nearly identical to that of Koodo. Your U.S. Galaxy S8 should work with Koodo.

Koodo operates on:

Bands 12/13/17/29 (or 700 MHz A/B/C/E) - LTE (rural coverage)

Bands 5 & 2 (or 850 MHz & 1900 MHz) - 3G/4G

Band 4 (or 1700/2100 MHz) - LTE

Band 7 (or 2600 MHz) - LTE

The Bands and radio frequencies in bold refer to most of AT&T’s network specifications.

AT&T Network Specs - see heading Advanced Network Technology

Excellent! Thanks very much, you all!!